This invention relates to an improved high pressure turbine casing of the type used for steam turbines in steam turbine power generating systems or other turbine drive applications, and in particular, to a turbine casing that is readily adaptable to a plurality of different system configurations.
In general, a steam turbine power system or plant comprises a series connected arrangement of a steam generator which generates steam from water through heating such as by gas or oil, the steam turbine which converts the energy contained in the steam into rotary power, and an electric power generator or other drive equipment driven by the steam turbine. The steam turbine casing encloses stationary blades secured to the interior of the casing and rotating blades on the turbine shaft wherein the high pressure steam is permitted to expand through alternating arrays of stationary and rotating blades or stages to impart rotation to the shaft which is connected to drive the driven equipment.
The steam after passing through the turbine blades is exhausted through the turbine casing and conducted to a condenser for conversion back to water which is returned to the steam generator to complete the closed-loop fluid system.
The turbine casing is a high pressure casing which is typically cast from a steel alloy such as chromium molybdenum, which can withstand the high internal pressures which may exceed 1500 pounds per square inch. A particular steam generating system may include accessories such as one or more feedwater heaters which utilize steam extracted from the interior of the turbine casing to preheat the water being fed from the condenser to the steam generator. Industrial steam turbine power generating systems often require other steam extractions at selected or required steam pressure(s) from the turbine for use with associated industrial processes in the plant such as, for example, drying paper in a paper mill, or even steam for lower pressure associated steam turbines. Admission of steam from the steam generator through the inlet to the turbine casing must also be provided. In addition other plant processes may generate steam which can profitably be used by the steam turbine if admitted to the turbine at the appropriate pressure location. Since the pressure in a steam turbine decreases as it passes through the various turbine stages, different pressure taps are located axially along the turbine casing. Various system demands result in the need for turbine casings to meet various flow configurations, and to meet a plurality of variables such as condensing or non-condensing exhaust; straight through, or controlled extraction or controlled admission; and from zero to four uncontrolled extractions or uncontrolled admissions.
The result is that the turbine casing must securely contain the high pressure steam used to drive the turbine rotor while at the same time allow for a variety of flow configurations and axial taps to meet the specific customer applications, needs, and design. The requirement for a pressure secure casing has resulted in the costly custom casting of casings for each application after it has been defined. However, because of the demanding technical requirements and extremely large size of typical industrial turbine casings, the customized alloy steel casings have typically required up to seven months for delivery. Cycles of an additional six to seven months are typically required to do the necessary machining, assembly, and test of the complete steam turbine, resulting in a fourteen month turbine delivery cycle which is frequently not responsive to customer needs.
Casting a hole through a double walled chamber in a marine steam turbine has been done to enable the location of an uncontrolled extraction or admission from or to the interior of the turbine. However, the need for controlled extractions or admissions and customizing flexibility in the design of industrial turbine applications has not enabled such designs to be stockpiled and later customized to meet industrial requirements.
As a result, and notwithstanding the competitive pressures and customer demand to expedite the turbine delivery cycle, the need for custom high pressure turbine casings has continued to heavily contribute to the long steam turbine delivery cycle over the years. Efforts such as extensive overtime in the manufacturing cycle has greatly increased cost and lowered production efficiency. Attempts to standardize industrial turbine casings to enable use of a standard casing for different industrial customers and applications has proved to be unworkable because of the many variables involved. The steam turbine industry has not been able to effectively cut the long lead time required to fabricate high pressure turbine casings in advance of specific customer orders. Moreover, the high cost of fabricating such casings makes it extremely risky to commence such fabrication in advance of a firm contract with definitive specifications for a turbine system, such that proceeding on the basis of perceived customer needs in advance of such a contract entails great risk that either the potential customer requirements might change or that the potential customer might place its order with a competitor.